diff --git a/test/intl402/Locale/prototype/getWeekInfo/likely-subtags-region.js b/test/intl402/Locale/prototype/getWeekInfo/likely-subtags-region.js
new file mode 100644
index 00000000000..0f7669a75f1
--- /dev/null
+++ b/test/intl402/Locale/prototype/getWeekInfo/likely-subtags-region.js
@@ -0,0 +1,75 @@
+// Copyright 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-intl.locale.prototype.getWeekInfo
+description: >
+  When the locale has no region, the Add Likely Subtags algorithm is used to
+  derive the region whose week info is returned.
+info: |
+  RegionPreference ( locale )
+  ...
+  2. If _region_ is *undefined*, then
+    a. Set _region_ to CanonicalUnicodeSubdivision(_locale_, *"sd"*).
+    b. If _region_ is *undefined*, then
+      i. Let _maximal_ be the result of the Add Likely Subtags algorithm applied
+         to _locale_. If an error is signaled, set _maximal_ to _locale_.
+      ii. Set _maximal_ to CanonicalizeUnicodeLocaleId(_maximal_).
+      iii. Set _region_ to GetLocaleRegion(_maximal_).
+      iv. If _region_ is *undefined*, then
+        1. Set _region_ to *"001"*.
+  ...
+features: [Intl.Locale, Intl.Locale-info]
+---*/
+
+// In order not to rely on a particular implementation's locale data, this test
+// searches for a suitable locale without a region subtag, but where the likely
+// region would influence the supported week info.
+//
+// Each candidate below is a language subtag together with the region that the
+// Add Likely Subtags algorithm derives for it (e.g. "th" maximizes to
+// "th-Thai-TH", so "th-TH"). A language tag without a region, subdivision ("sd"
+// keyword), or region override ("rg" keyword) passed to RegionPreference must
+// apply Add Likely Subtags and arrive at the paired region. Its week info must
+// therefore be identical to those of the candidate locale. An incorrect
+// implementation might instead fall back to the region "001" in step 2.b.iv.
+//
+// We could assume even less about the locale data by not hardcoding these
+// likely regions and instead checking all available locales with the result of
+// maximize() on each one, but that assumes the implementation has a working
+// maximize(), and if that's the case then it probably implements this step
+// correctly as well.
+function weekInfoEqual(a, b) {
+  return a.firstDay === b.firstDay && compareArray(a.weekend, b.weekend);
+}
+
+function findSuitableTestData() {
+  const candidates = ["th-TH", "fa-IR", "ja-JP", "sa-IN", "ps-AF"];
+
+  for (const regionTag of candidates) {
+    const weekInfoWithLikelyRegion = new Intl.Locale(regionTag).getWeekInfo();
+
+    const bareLanguage = regionTag.replace(/-.*/, "");
+    const fallbackLocale = new Intl.Locale(`${bareLanguage}-001`);
+    if (!weekInfoEqual(fallbackLocale.getWeekInfo(), weekInfoWithLikelyRegion)) {
+      return [bareLanguage, weekInfoWithLikelyRegion];
+    }
+  }
+
+  assert(false, "No suitable test data found in this implementation. Consider updating the candidate list");
+}
+
+const [language, expectedWeekInfo] = findSuitableTestData();
+
+const locale = new Intl.Locale(language);
+const weekInfo = locale.getWeekInfo();
+assert.sameValue(
+  weekInfo.firstDay,
+  expectedWeekInfo.firstDay,
+  `getWeekInfo() for "${language}" should return firstDay that matches the likely region`
+);
+assert.compareArray(
+  weekInfo.weekend,
+  expectedWeekInfo.weekend,
+  `getWeekInfo() for "${language}" should return weekend that matches the likely region`
+);
diff --git a/test/intl402/Locale/prototype/getWeekInfo/region-override.js b/test/intl402/Locale/prototype/getWeekInfo/region-override.js
new file mode 100644
index 00000000000..3a5e78ce887
--- /dev/null
+++ b/test/intl402/Locale/prototype/getWeekInfo/region-override.js
@@ -0,0 +1,79 @@
+// Copyright 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-intl.locale.prototype.getWeekInfo
+description: >
+  When the locale has a region override ("rg") keyword, its region overrides
+  the region subtag when deriving the week info.
+info: |
+  WeekInfoOfLocale ( loc )
+  ...
+  2. Let _preference_ be RegionPreference(_loc_.[[Locale]]).
+  3. Let _region_ be _preference_.[[Region]].
+  4. Let _regionOverride_ be _preference_.[[RegionOverride]].
+  5. If _regionOverride_ is not *undefined* and week data for _regionOverride_
+     are available, then
+    a. Let _lookupRegion_ be _regionOverride_.
+  6. Else,
+    a. Let _lookupRegion_ be _region_.
+  ...
+
+  RegionPreference ( locale )
+  ...
+  3. Let _regionOverride_ be CanonicalUnicodeSubdivision(_locale_, *"rg"*).
+  4. Return { [[Region]]: _region_, [[RegionOverride]]: _regionOverride_ }.
+features: [Intl.Locale, Intl.Locale-info]
+---*/
+
+// In order not to rely on a particular implementation's locale data, this test
+// searches for a suitable locale with a region subtag together with a region
+// override ("rg" extension) whose region would influence the supported week
+// info.
+//
+// Each candidate below is a locale with a region override extension, together
+// with a locale that names the override's region explicitly (e.g.
+// "en-US-u-rg-inzzzz" overrides the region with "IN", so "en-IN"). Because the
+// region override is set, WeekInfoOfLocale must use it as the lookup region
+// instead of the region subtag. Its week info must therefore be identical to
+// those of the paired override-region locale. An incorrect implementation might
+// instead ignore the "rg" keyword and use the region subtag.
+function weekInfoEqual(a, b) {
+  return a.firstDay === b.firstDay && compareArray(a.weekend, b.weekend);
+}
+
+function findSuitableTestData() {
+  const candidates = [
+    ["en-US-u-rg-dezzzz", "en-DE"],
+    ["en-US-u-rg-inzzzz", "en-IN"],
+    ["en-US-u-rg-irzzzz", "en-IR"],
+    ["en-US-u-rg-afzzzz", "en-AF"],
+  ];
+
+  for (const [overrideTag, regionTag] of candidates) {
+    const weekInfoWithOverrideRegion = new Intl.Locale(regionTag).getWeekInfo();
+
+    const baseName = overrideTag.replace(/-u-.*/, "");
+    const baseLocale = new Intl.Locale(baseName);
+    if (!weekInfoEqual(baseLocale.getWeekInfo(), weekInfoWithOverrideRegion)) {
+      return [overrideTag, weekInfoWithOverrideRegion];
+    }
+  }
+
+  assert(false, "No suitable test data found in this implementation. Consider updating the candidate list");
+}
+
+const [overrideTag, expectedWeekInfo] = findSuitableTestData();
+
+const locale = new Intl.Locale(overrideTag);
+const weekInfo = locale.getWeekInfo();
+assert.sameValue(
+  weekInfo.firstDay,
+  expectedWeekInfo.firstDay,
+  `getWeekInfo() for "${overrideTag}" should return firstDay that matches the override region`
+);
+assert.compareArray(
+  weekInfo.weekend,
+  expectedWeekInfo.weekend,
+  `getWeekInfo() for "${overrideTag}" should return weekend that matches the override region`
+);
diff --git a/test/intl402/Locale/prototype/getWeekInfo/region-priority.js b/test/intl402/Locale/prototype/getWeekInfo/region-priority.js
new file mode 100644
index 00000000000..0b4cb8fb832
--- /dev/null
+++ b/test/intl402/Locale/prototype/getWeekInfo/region-priority.js
@@ -0,0 +1,97 @@
+// Copyright 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-intl.locale.prototype.getWeekInfo
+description: >
+  The region used to look up the week info is chosen from the available signals
+  in the correct priority order.
+info: |
+  WeekInfoOfLocale ( loc )
+  ...
+  2. Let _preference_ be RegionPreference(_loc_.[[Locale]]).
+  3. Let _region_ be _preference_.[[Region]].
+  4. Let _regionOverride_ be _preference_.[[RegionOverride]].
+  5. If _regionOverride_ is not *undefined* and week data for _regionOverride_
+     are available, then
+    a. Let _lookupRegion_ be _regionOverride_.
+  6. Else,
+    a. Let _lookupRegion_ be _region_.
+  ...
+
+  RegionPreference ( locale )
+  1. Let _region_ be GetLocaleRegion(_locale_).
+  2. If _region_ is *undefined*, then
+    a. Set _region_ to CanonicalUnicodeSubdivision(_locale_, *"sd"*).
+    b. If _region_ is *undefined*, then
+      i. Let _maximal_ be the result of the Add Likely Subtags algorithm applied
+         to _locale_. If an error is signaled, set _maximal_ to _locale_.
+      ii. Set _maximal_ to CanonicalizeUnicodeLocaleId(_maximal_).
+      iii. Set _region_ to GetLocaleRegion(_maximal_).
+      iv. If _region_ is *undefined*, then
+        1. Set _region_ to *"001"*.
+  3. Let _regionOverride_ be CanonicalUnicodeSubdivision(_locale_, *"rg"*).
+  4. Return { [[Region]]: _region_, [[RegionOverride]]: _regionOverride_ }.
+features: [Intl.Locale, Intl.Locale-info]
+---*/
+
+// Together, WeekInfoOfLocale and RegionPreference select the region used to
+// look up the week info from the available signals in this priority order
+// (highest first):
+//
+//   1. the "rg" region override keyword (when week data for it exists)
+//   2. the region subtag
+//   3. the "sd" subdivision keyword
+//   4. the region computed by the Add Likely Subtags algorithm
+//   5. the "001" (world) region
+//
+// Each entry below is a locale that carries the signal for its priority level
+// on top of all lower-priority signals, paired with a locale that names the
+// region that signal must resolve to. The paired locale uses the same base
+// language so that any language-dependent locale data is held constant. For
+// example, "fa-JP-u-sd-inka-rg-afzzzz" carries an "rg" override of region "AF",
+// a region subtag "JP", an "sd" subdivision in region "IN", and a base language
+// "fa" whose likely region is "IR"; the "rg" override has the highest priority,
+// so the locale must behave like "fa-AF".
+//
+// The last entry has no lower-priority signals: the language "eo" (Esperanto)
+// has no likely region, so its region falls back to "001".
+const levels = [
+  { description: 'the "rg" region override', locale: "fa-JP-u-sd-inka-rg-afzzzz", region: "fa-AF" },
+  { description: "the region subtag", locale: "fa-JP-u-sd-inka", region: "fa-JP" },
+  { description: 'the "sd" subdivision', locale: "fa-u-sd-inka", region: "fa-IN" },
+  { description: "the Add Likely Subtags region", locale: "fa", region: "fa-IR" },
+  { description: 'the "001" region', locale: "eo", region: "eo-001" },
+];
+
+function weekInfoEqual(a, b) {
+  return a.firstDay === b.firstDay && compareArray(a.weekend, b.weekend);
+}
+
+// For each priority level to be observable, the region it selects must have
+// different week info than the region selected by the next lower level in this
+// implementation; otherwise the test could not tell them apart.
+for (let i = 0; i < levels.length - 1; i++) {
+  const higher = new Intl.Locale(levels[i].region).getWeekInfo();
+  const lower = new Intl.Locale(levels[i + 1].region).getWeekInfo();
+  assert(
+    !weekInfoEqual(higher, lower),
+    `Inconclusive: ${levels[i].description} and ${levels[i + 1].description} ` +
+    "selected regions with identical week info in this implementation. Consider updating the test data"
+  );
+}
+
+for (const { description, locale, region } of levels) {
+  const weekInfo = new Intl.Locale(locale).getWeekInfo();
+  const expected = new Intl.Locale(region).getWeekInfo();
+  assert.sameValue(
+    weekInfo.firstDay,
+    expected.firstDay,
+    `getWeekInfo() for "${locale}" should use ${description}, like "${region}": firstDay mismatch`
+  );
+  assert.compareArray(
+    weekInfo.weekend,
+    expected.weekend,
+    `getWeekInfo() for "${locale}" should use ${description}, like "${region}": weekend mismatch`
+  );
+}
diff --git a/test/intl402/Locale/prototype/getWeekInfo/subdivision-region.js b/test/intl402/Locale/prototype/getWeekInfo/subdivision-region.js
new file mode 100644
index 00000000000..4722902e8e2
--- /dev/null
+++ b/test/intl402/Locale/prototype/getWeekInfo/subdivision-region.js
@@ -0,0 +1,72 @@
+// Copyright 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-intl.locale.prototype.getWeekInfo
+description: >
+  When the locale has no region but has a subdivision ("sd") keyword, the
+  subdivision's region is used to derive the week info.
+info: |
+  RegionPreference ( locale )
+  1. Let _region_ be GetLocaleRegion(_locale_).
+  2. If _region_ is *undefined*, then
+    a. Set _region_ to CanonicalUnicodeSubdivision(_locale_, *"sd"*).
+    b. If _region_ is *undefined*, then
+      i. Let _maximal_ be the result of the Add Likely Subtags algorithm applied
+         to _locale_. If an error is signaled, set _maximal_ to _locale_.
+      ...
+  ...
+features: [Intl.Locale, Intl.Locale-info]
+---*/
+
+// In order not to rely on a particular implementation's locale data, this test
+// searches for a suitable locale without a region subtag, but with a
+// subdivision ("sd" extension) whose region would influence the supported week
+// info.
+//
+// Each candidate below is a locale with a subdivision extension but no region
+// subtag, together with a locale that names that subdivision's region
+// explicitly (e.g. "en-u-sd-inka" has subdivision "inka", whose region is "IN",
+// so "en-IN"). Because such a locale has no region subtag, RegionPreference
+// must derive its region from the subdivision. Its week info must therefore be
+// identical to those of the paired region locale. An incorrect implementation
+// might instead ignore the "sd" extension and apply Add Likely Subtags to the
+// bare language, giving "en-Latn-US".
+function weekInfoEqual(a, b) {
+  return a.firstDay === b.firstDay && compareArray(a.weekend, b.weekend);
+}
+
+function findSuitableTestData() {
+  const candidates = [
+    ["en-u-sd-inka", "en-IN"],
+    ["en-u-sd-irthr", "en-IR"],
+    ["en-u-sd-afgh", "en-AF"],
+  ];
+
+  for (const [subdivisionTag, regionTag] of candidates) {
+    const weekInfoWithSubdivisionRegion = new Intl.Locale(regionTag).getWeekInfo();
+
+    const baseName = subdivisionTag.replace(/-u-.*/, "");
+    const fallbackLocale = new Intl.Locale(baseName).maximize();
+    if (!weekInfoEqual(fallbackLocale.getWeekInfo(), weekInfoWithSubdivisionRegion)) {
+      return [subdivisionTag, weekInfoWithSubdivisionRegion];
+    }
+  }
+
+  assert(false, "No suitable test data found in this implementation. Consider updating the candidate list");
+}
+
+const [subdivisionTag, expectedWeekInfo] = findSuitableTestData();
+
+const locale = new Intl.Locale(subdivisionTag);
+const weekInfo = locale.getWeekInfo();
+assert.sameValue(
+  weekInfo.firstDay,
+  expectedWeekInfo.firstDay,
+  `getWeekInfo() for "${subdivisionTag}" should return firstDay that matches the subdivision's region`
+);
+assert.compareArray(
+  weekInfo.weekend,
+  expectedWeekInfo.weekend,
+  `getWeekInfo() for "${subdivisionTag}" should return weekend that matches the subdivision's region`
+);